Assessment of herbicides for use on the sweet herb, Stevia rebaudiana in the tropics

The leaves and stems of Stevia rebaudiana produce steviol glycosides which are sweeter than sucrose, of low calorific value, pH- and heat-stable, and may have a variety of therapeutic benefits. Stevia seed germination is poor and the initial stages of growth are slow, which makes young plants susceptible to competition from weed species. In Australia, no herbicides are currently registered for use on Stevia. The purpose of this study was to assess the efficacy of 16 herbicides on S. rebaudiana and associated weeds. All 16 herbicides were trialled at their most relevant application rates at a field site in tropical Queensland. The best four performers were then tested at two rates of application at another tropical site. The post-planting, pre-weed emergence, oryzalin-containing herbicide, Prolan 500, at either 6.8 or 8.5 l ha–1 provided the best weed control without affecting the biomass of the Stevia crop. High-performance liquid chromatography was used to determine if rebaudioside A and stevioside concentrations were affected in leaves treated with Prolan 500. It was found that at 1, 4, and 8 weeks after treatment, 8.5 l ha–1 Prolan 500 resulted insignificantly lower rebaudioside A and stevioside concentrations than applying Prolan 500 at 6.8 l ha–1. We therefore recommend that the thoryzalin-containing herbicide, Prolan 500 is suitable for use on S. rebaudiana at an application rate of 6.8 l ha–1.